The present invention generally relates to a recording medium of an optical reading and recording apparatus which is adapted to record, read or erase information with the use of optical means such as laser beams or the like.
The construction of the conventional optical disk substrate is basically shown in FIG. 9.
Reference numeral 41 is a plastic substrate, such as polycarbonate, polyolefin, acryl, epoxy resins or the like, provided with grooves for tracking guide of optical spots. A dielectric layer 42 of SiO.sub.2, ZnS or the like, a reading and recording layer 43 of TbFeCo, DyFeCo or the like, and a protective layer 44 of 2MgO.SiO.sub.2, ZnS.SiO.sub.2, SiN or the like are laminated sequentially on the plastic substrate. An overcoat layer 45 of epoxide acrylate, epoxy resin, acrylic resin or the are further coated on it. A hard coat layer 46 of epoxy acrylate, epoxy resin, acrylic resin or the like are further coated on the beam incident on the side of the plastic substrate 41.
When the reading and recording layer 43 of TbFeCo, DyFeCo or the like is as small as 50 nm or lower in film thickness, a reflection layer composed of A1, Cu or the like is provided between the reading and recording layer 43 and the protective layer 44 so as to increase the reflection light intensity.
An optical recording medium as described hereinabove is heated to approximately a temperature resulting in smaller local coercive force or near a compensation temperature of approximately curie temperature or by the laser beam irradiation upon a perpendicular magnetization film, in a case of a magneto-optical recording medium using a perpendicular magnetization film having a magnetic optical effect as the reading and recording layer 43, so as to magnetize it in the direction of a bias field for effecting a thermomagnetic recording operation. In order to effect a reading operation of the recording signal, a laser beam smaller in intensity than the laser beam at a recording and erasing time is irradiated and the rotation (magnetic optical effects of so-called kerr effect and faraday effect) of a polarization plane of reflection beams or transmission beams are detected, in accordance with a direction of the magnetization of the perpendicular magnetization film, as the intensity variation of the beams with the use of an analyzer.
In a case of an optical disk where a phase-change optical recording medium and an organic material are used for the reading and recording layer 43, optical beams are similarly irradiated upon the medium so as to cause local temperature rises or chemical variation by optical absorption. In order to read the signal, optical beams different in intensity or wavelength from a recording time are irradiated in local change on the medium caused by the recording operation so as to detect the reflection beams or the transmission beams.
Further, as a method of effecting a high density reading and recording operation, a high density reading and recording operation of 0.6 .mu.m or lower in reading domain length along by track can be effected from one portion of a temperature raised portion of spots of laser beams by the use of an exchange coupled multi-layer film of rare-earth-transition-metal alloy system thin film in a reading and recording layer 43. (Japanese Patent Laid-Open Publication Hei No. 3-93056, Japanese Patent Laid-Open Publication Hei No. 3-93058.)
A reproducing principle by a super resolution system (hereinafter referred to as RAD system) called RAD will be described briefly in a case where high density reading operation is effected with the use of reading and recording layer 43 composed of exchange coupled two layer films of a recording film a reading film for making a reading signal amount of 0.6 .mu.m or lower in domain length as one example. In the RAD system, an initial magnetic field is disposed immediately before the laser spot and the magnetization direction of the reading film is arranged in a direction of the initial magnetic field. At this time, the reading film operates as a mask. When a disk is rotated so as to pass a laser spot at the reading time, the temperature of the read film rises gradually and a high temperature area behind the laser spot is considerably lowered in the coercive force of the reading layer. When the total of the exchange coupling force with respect to the read film and the bias field at the reading time becomes larger than the coercive force of the reading layer, the magnetization of the recording film is copied. Information of a recording film is read from a portion out of a mask of the reading layer. A direction of a bias field is a direction opposite to the initial field. After the laser beams have passed, the temperature of the reading and recording layer is lowered. So the domain of the recording film is copied on the reading layer so as to return to the original condition. In the magneto-optical recording medium of this construction, the recording and reading operations of 0.5 .mu.m or lower in domain length can be effected.
An optical recording medium of such conventional construction has a problem in that a recording power range to be tolerated is small, because crosstalks with respect to the adjacent tracks become larger in a case of a disk small in track pitch because of the expanse, in a radial direction, of the recording magnetic section when a recording operation has been effected with excessive power. When the recording power range is expanded, information of the adjacent tracks is erased because of cross erasing and the track pitch cannot be made smaller, with a problem that the recording density cannot be made larger.
When the reading operation is effected with the use of a super resolution system of RAD or the like, it is necessary to adjust the intensity of the laser beams, the strength of the bias field, the area to be exchange coupled. in order to effect a high recording density operation. It is necessary to make the laser spot center conform to the central position of the temperature distribution so as to obtain sufficient reading signals in a wide range. But there is a problem that the high recording density operation is hard to effect so that the central position of the temperature distribution is moved in a progressing direction of a disk with respect to the laser spot center by the thermal conductivity in the reading and recording layer.